Calculations show 2D boron is low-temperature superconductor

5 April 2016

Electrons with opposite momenta and spins pair up via lattice vibrations at low temperatures in two-dimensional boron to give it superconducting properties, according to new research by theoretical physicists at Rice University. Image: Evgeni Penev/Rice University.

Scientists at Rice University have determined that two dimensional (2D) boron is a natural low-temperature superconductor. In fact, it may be the only 2D material with such potential.

Rice theoretical physicist Boris Yakobson and his co-workers have published calculations in Nano Lettersshowing that atomically flat boron is metallic and will transmit electrons with no resistance. As with most superconducting materials, the hitch is that 2D boron only loses its resistivity at very low temperatures, in this case between 10K and 20K (-253°C to-263°C). But for making very small superconducting circuits, it might be the only game in town.

The basic phenomenon of superconductivity has been known for more than 100 years, explained Evgeni Penev, a research scientist in the Yakobson group, but its presence had not been tested for in atomically flat boron.

"It's well-known that the material is pretty light because the atomic mass is small," Penev said. "If it's metallic too, these are two major prerequisites for superconductivity. That means at low temperatures, electrons can pair up in a kind of dance in the crystal."

"Lower dimensionality is also helpful," added Yakobson. "It may be the only, or one of very few, two-dimensional metals. So there are three factors that gave the initial motivation for us to pursue the research. Then we just got more and more excited as we got into it."

Electrons with opposite momenta and spins effectively become what are known as Cooper pairs; they attract each other at low temperatures with the help of lattice vibrations known as phonons, giving the material its superconducting properties, Penev said. "Superconductivity becomes a manifestation of the macroscopic wave function that describes the whole sample. It's an amazing phenomenon," he said.

It wasn't entirely by chance that the first theoretical paper establishing conductivity in a 2D material appeared at roughly the same time as the first samples of the material were made by laboratories in the US and China. In fact, an earlier paper by the Yakobson group had offered a road map for doing so.

That 2D boron has now been produced is a good thing, according to Yakobson and lead authors Penev and Alex Kutana, a postdoctoral researcher at Rice. "We've been working to characterize boron for years, from cage clusters to nanotubes to planer sheets, but the fact that these papers appeared so close together means these labs can now test our theories," Yakobson said.

"In principle, this work could have been done three years ago as well," he said. "So why didn't we? Because the material remained hypothetical; okay, theoretically possible, but we didn't have a good reason to carry it too far.

"But then last fall it became clear from professional meetings and interactions that it can be made. Now those papers are published. When you think it's coming for real, the next level of exploration becomes more justifiable," Yakobson said.

Boron atoms can make more than one pattern when coming together as a 2D material, another characteristic predicted by Yakobson and his team that has now been proved correct. These patterns, known as polymorphs, may allow researchers to tune the material's conductivity "just by picking a selective arrangement of the hexagonal holes," Penev said.

He also noted that boron's qualities were hinted at when researchers discovered more than a decade ago that magnesium diborite is a high-temperature electron-phonon superconductor. "People realized a long time ago the superconductivity is due to the boron layer," Penev said. "The magnesium acts to dope the material by spilling some electrons into the boron layer. In this case, we don't need them because the 2D boron is already metallic." He added that isolating 2D boron between layers of inert hexagonal boron nitride (aka ‘white graphene’) might help stabilize its superconducting nature.

Without the availability of a block of time on several large government supercomputers, the study would have taken a lot longer, Yakobson said. "Alex did the heavy lifting on the computational work," he said. "To turn it from a lunchtime discussion into a real quantitative research result took a very big effort."

The paper is the first by Yakobson's group on the topic of superconductivity, though Penev is a published author on the subject. "I started working on superconductivity in 1993, but it was always kind of a hobby, and I hadn't done anything on the topic in 10 years," Penev said. "So this paper brings it full circle."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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